1. Field of the Invention
This invention relates to a process of producing a resin sheet or film from thermoplastic resin and to equipment therefore.
2. Description of the Prior Art
Crystalline thermoplastic resins such as polypropylene and polyethylene have been utilized in various packaging fields and the like as sheets and containers due to their easiness in being thermoformed.
In producing crystalline thermoplastic resin sheets as represented by polypropylene as described above, to improve the transparency of these sheets, it has been practice that the molten resin sheet (melt web) be quenched to control the crystalline structure. In a quenching process of this type, water cooling is the most efficient process. However, this process presents a problem in that it is difficult to cool uniformly a molten film-shaped resin (melt web) extruded from an extrusion die. The inventors of the present invention have proposed a process of using a multi-stage slit, through which cooling water flows down, as a process of uniformly water-cooling the molten film-shaped resin (Japanese Patent Kokai (Laid-Open) No. 58-203018).
However, even if the cooling conditions are controlled, the cooling effect thereof is naturally limited, whereby the transparency, surface uniformity of the resin sheet, in the molded items, have not always been satisfactory.
As the result of various studies conducted by the inventors of the present invention, it was found that conditions of extrusion have a great influence at a stage of molten film-shaped resin, where the molten film-shaped resin extruded has not yet come into contact with the cooling water, i.e. at a stage covering a range from feeding a material to a resin extruder to an extrusion die. Based on this, a process of using an extruder provided at the forward end portion thereof with a stress relaxing section as one of the above-described conditions of extrusion is devised, and a process, in which the above-described conditions of extrusion are combined with a slit water cooling method in use, has also been proposed (Patent Kokai (Laid-Open) No. 62-146611 and patent application No. 61-202146). These processes are outstanding ones, but, present such disadvantages that the selection of the devices is limited, and it is difficult to combinedly satisfy the kneading properties (extensive mixing, dispersing), stress relaxing properties and extrusion stability.
To solve the problems between the kneading properties and stress relaxing properties, and between the high speed molding properties and low temperature extrusion properties, there has been known a tandem extruder (cascade extruder) constituted by two extruders (Patent Kokai (Laid-Open) No. 59-5044).
This tandem extruder is constructed such that a first and a second extruders are serially connected to each other through a connecting pipe, and pressure detectors are provided on an outlet side of the second extruder and the connecting pipe, respectively, whereby the screw speed of the first extruder is controlled so that a difference between pressures detected by these pressure detectors can be held at a predetermined relationship.
With the above-described tandem extruder, in general, there are offered the advantages that the extruders can be designed such that the first extruder performs plasticization and kneading of the resin and the second extruder carries out mixing (distribution) resin temperature uniformity and metered extrusion, to thereby meet the respective functions of the extruders.
However, the first extruder mainly aims at plasticization and kneading, but, in actuality, the dispersing properties, kneading properties, resin temperature uniformity properties and the like cannot be expected so much therefrom. For this reason, the resin is extruded from the outlet of the first extruder in a state where the kneading (dispersion) and resin temperature uniformity are unsatisfactory, and, in order to produce a satisfactory sheet, it is necessary to further carry out the dispersion, kneading (dispersion), mixing (distribution) and resin temperature uniformity in the second extruder.
Accordingly, in the second extruder, shear stress and resin temperature become high, with the result that the resin, being low in temperature and low in residual stress, cannot be extruded, so that a sheet having excellent transparency and glossiness, etc. cannot be obtained.